Remove references to filenames.json

The use of this file has been deprecated in favor of other
Datasource methods.

Removed reference to filenames.json in OH, MD and ID datasources.

Removed the ``clear_filenames()``, ``filename_mappings()`` and
``update_mappings()`` methods from ``base.datasource.Datasource``.
These methods referenced ``filenames.json``.  As far as I could tell
with a quick grep, these weren't being called from anywhere.

Closes https://github.com/openelections/core/issues/112